Registered microperforated films for modified/controlled atmosphere packaging

ABSTRACT

A microperforated packaging material for use in modifying or controlling the flow of oxygen and carbon dioxide into and/out of a fresh produce container, where the microperforations are specifically tailored in size, location and number for the specific produce. The packaging system specifically tailors microperforated containers and packaging for particular produce to optimally preserve the produce, using a method of making registered microperforations on the packaging material using a CO 2  laser and a sensor mechanism.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) from U.S.patent application Ser. No. 60/132,388 filed on May 4, 1999, and is adivisional of U.S. patent application Ser. No. 09/528,290 filed Mar. 17,2000, now U.S. Pat. No. 6,441,340, hereby incorporated by reference forall purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of packaging for respiring orbiochemically active agricultural products and commodities such as freshfruits, fresh vegetables, fresh herbs, and flowers (herein referred tocollectively as produce or fresh produce) and more particularly toregistered microperforations in packaging materials for use in modifyingor controlling the flow of oxygen and carbon dioxide into and/out of afresh produce container.

BACKGROUND OF THE INVENTION

The quality and shelf life of many food products is enhanced byenclosing them in packaging that modifies or controls the atmospheresurrounding the product. Increased quality and longer shelf life resultin fresher products for the consumer, less waste from spoiled produce,better inventory control, and appreciable overall savings for the foodindustry at both the retail and wholesale levels.

Modified atmosphere packaging (MAP) and controlled atmosphere packaging(CAP) are often used interchangeably in the industry, and much confusionexists on their exact meanings. Both refer to methods to control theatmosphere in the package. In the processed foods area, MAP isconsidered a static method for controlling the atmosphere whereby aninitial charge of a specific gas composition, e.g. 30% CO₂ and 70% N₂,is introduced into a barrier container before sealing.

The oxygen transmission rate (OTR) of a film is expressed as ccO₂/m²-day-atmosphere, where one atmosphere is 101325 kg/ms². Generally,a barrier container is one that has an OTR of <70 cc/m²-day-atm. Theunits describing the flow of a particular gas through a film are “flux”,expressed as cc/day-atm.

For fresh produce, the primary means to extend quality and shelf life istemperature control. However, more than 50 years of evidence fromindustry practices on bulk storage of fresh fruits and vegetables inrefrigerated controlled atmosphere storage rooms has shown thatatmosphere control can contribute greatly to quality retention and shelflife. The use of MAP/CAP for fresh produce was a natural progressiononce packaging technology had advanced to include the production ofnon-barrier (often referred to in the industry as “breathable”)materials.

The goal in fresh fruit and vegetable packaging is to use MAP/CAP topreserve produce quality by reducing the aerobic respiration rate butavoiding anaerobic processes that lead to adverse changes in texture,flavor, and aroma, as well as an increased public health concern.Aerobic respiration can be defined by the following equation:(CH₂O)n+nO₂→nCO₂+nH₂O+heatwhere O₂ from the air is used to metabolize carbohydrate ((CH₂O)n)reserves and in the process, CO₂, and H₂O are produced and heat isgenerated. For each respiring item, there is an optimum O₂ and CO₂ levelthat will reduce its respiration rate and thereby, slow aging anddegradative processes. Different fresh produce items have differentrespiration rates and different optimum atmospheres for extendingquality and shelf life.

The concept of passive MAP became common with the development ofpackaging materials with OTRs of 1085 to 7000 cc/m²-day-atm forfresh-cut salads. In passive MAP, the produce is sealed in packages madefrom these low barrier materials and allowed to establish its ownatmosphere over time through produce respiration processes. Sometimesthe package is gas-flushed with N₂ or a combination of CO₂ and N₂, orO₂, CO₂, and N₂ before sealing to rapidly establish the desired gascomposition inside the package. Alternately, a portion of the air may beremoved from the pack, either by deflation or evacuation, before thepackage is sealed, to facilitate rapid establishment of the desired gascontent.

In CAP, the atmosphere in the package is controlled at well-definedlevels throughout storage. CAP can take many forms, and may even involveenclosing gas absorber packets inside processed food barrier packages.For example, CO₂ absorber sachets may be sealed inside coffee containersto absorb and control the level of CO₂ that continues to be generated bythe ground coffee. Sachets containing iron oxides are enclosed inbarrier packages of fresh refrigerated pasta to absorb low levels of O₂entering the package through the plastic material.

CAP of fresh produce is just a more controlled version of MAP. Itinvolves a precise matching of packaging material gas transmission rateswith the respiration rates of the produce. For example, many fresh-cutsalad packages use passive MAP as described herein. If the packages aretemperature-abused (stored at 6–10° C. or higher), O₂ levels diminish toless than 1%, and CO₂ levels can exceed 20%. If these temperature-abusedpackages are then placed back into recommended 3–4° C. storage, thepackaging material gas transmission rates may not be high enough toestablish an aerobic atmosphere (<20% CO₂, >1–2% O₂) so fermentationreactions cause off-odors, off-flavors, and slimy product. If the saladwas in a CAP package, the O₂ levels would decrease and CO₂ levelsincrease with temperature abuse, but would be re-established to desiredlevels within a short time after the product is returned to 4° C.storage temperatures.

Today, films made from polymer blends, coextrusions, and laminatematerials with OTRs of 1085 to 14,000 cc/100 m²-day-atm are being usedfor packaging various weights of low respiring produce items likelettuce and cabbage. These OTRs, however, are much too low to preservethe fresh quality of high respiring produce like broccoli, mushrooms,and asparagus. In addition, existing packaging material OTRs for bulkquantities (>1 kg) of some low respiring produce are not high enough toprevent sensory quality changes during storage. Therefore, severalapproaches have been patented describing methods to produce packagingmaterials to accommodate the higher respiration rate requirements andhigher weights of a wide variety of fresh produce items.

U.S. Pat. No. 4,842,875, U.S. Pat. No. 4,923,703, U.S. Pat. No.4,910,032, U.S. Pat. No. 4,879,078, and U.S. Pat. No. 4,923,650 describethe use of a breathable microporous patch placed over an opening in anessentially impermeable fresh produce container to control the flow ofoxygen and carbon dioxide into and out of the container during storage.Although this method works effectively, the breathable patch must beproduced by normal plastic extrusion and orientation processes, whereby,a highly filled, molten plastic is extruded onto a chill roll andoriented in the machine direction using a series of rolls that decreasethe thickness of the web. During orientation, micropores are created inthe film at the site of the filler particles. Next, the microporous filmmust be converted into pressure sensitive adhesive patches or heat-sealcoated patches using narrow web printing presses that apply a pattern ofadhesive over the microporous web and die-cut the film into individualpatches on a roll. These processes make the cost of each patch tooexpensive for the wide spread use of this technology in the marketplace.In addition, the food packer has to apply the adhesive-coated breathablepatch over a hole made in the primary packaging material (bag or liddingfilm) during the food packaging operation. To do this, the packer mustpurchase hole-punching and label application equipment to install oneach packaging equipment line. These extra steps not only increasepackaging equipment costs, but also greatly reduce packaging speeds,increase packaging material waste, and therefore, increase totalpackaging costs.

An alternative to microporous patches for MAP/CAP of fresh fruits andvegetables is to microperforate polymeric packaging materials. Variousmethods can be used to microperforate packaging materials: cold or hotneedle mechanical punches, electric spark and lasers. Mechanical punchesare slow and often produce numerous large perforations (1 mm or larger)throughout the surface area of the packaging material, making itunlikely that the atmosphere inside the package will be modified belowambient air conditions (20.9% O₂, 0.03% CO₂). Equipment for sparkperforation of packaging materials is not practical for most plasticconverting operations, because the packaging material must be submergedin either an oil or a water bath while the electrical pulses aregenerated to microperforate the material. The most efficient andpractical method for making microperforated packaging materials is usinglasers.

UK Patent Application No. 2 200 618 A and European Patent ApplicationNo. 88301303.9 describe the use of a mechanical perforating method tomake perforations with diameters of 0.25 mm to 60 mm in PVC films forproduce packaging. Rods with pins embedded into the surface of thecylinder are used to punch holes in the film. For each produce item tobe packaged, the rod/pin configuration is manually changed so that thenumber of perforation rows in the film, the distance apart of the rows,the pitch of the pins used to make the holes, and the size of the holesare adjusted to meet the specific requirements of the produce. Theproduce requirements are determined by laboratory testing produce packedin a variety of perforated films. The invention does not disclose anymathematical method to determine the appropriate size or number ofperforations to use with different produce items. In addition, the holesizes, 20 mm to 60 mm, which are claimed, would be too large toeffectively control the atmosphere inside packages containing less thanseveral kilograms of produce. Furthermore, the complicated perforationmethod would cause lost package production time due to equipment(perforation cylinder) change-overs for different perforation patterns.In addition, the invention cautions that the produce should be placed inthe package so that the perforations are not occluded and care should betaken to prevent taping over the perforations in the film. Since theperforations are not registered in a small area on the package, but areplaced throughout the main body of the plastic film, the likelihood ishigh that perforations will be occluded by the produce inside thepackage or by pressure sensitive adhesive labels applied on packages formarketing purposes. When holes are blocked, the principal route for gastransmission through the film is blocked which leads to anaerobicconditions and fermentative reactions. The result is poor sensoryproperties, reduced shelf life and possible microbiological safetyconcerns. Therefore, it is important that perforations be registered ina well-defined area of the package where the likelihood of theirocclusion during pack-out, storage, transportation, and retail displayis minimized.

U.S. Pat. No. 5,832,699, UK Patent Application 2 221 692 A, and EuropeanPatent Application 0 351 116 describe a method of packaging plantmaterial using perforated polymer films having 10 to 1000 perforationsper m² (1550 in²) with mean diameters of 40 to 60 microns but notgreater than 100 microns. The patents recommend the use of lasers forcreating the perforations, but do not describe the equipment orprocesses necessary to accomplish this task. The patents describe thelimits of the gas transmission rates of the perforated film: OTR nogreater than 200,000 cc/m²-day-atm (12,903 cc O₂/100 in²-day-atm), andMVTR no greater than 800 g/m²-day-atm (51.6 g/100 in²-day-atm). However,the OTR of a film does not define the total O₂ Flux (cc O₂/day-atm)needed by a fresh produce package to maintain a desired O₂ and CO₂internal atmosphere based on the respiration rate of the specificproduce item, the weight of the produce enclosed in the package, thesurface area of the package, and the storage temperature. A 50-micronperforation has a very small surface area (1.96×10⁻⁹ m²) and a low O₂Flux (about 80 cc/day-atm) compared to its very high OTR (>200,000 ccO₂/m²-day-atm). Therefore, one 50-micron perforation would exceed theOTR limit of this invention. Furthermore, fresh produce items such asfresh spinach are very susceptible to moisture that accumulates insidepackages so produce weights greater than 0.5 kg requires 2–3 times moremoisture vapor transmission than the upper limit described in thispatent.

The above inventions do not address the issue of microperforationocclusion by produce inside the package when microperforations areplaced throughout the length and width of the film. Since 20 to 100micron holes cannot be readily seen with the naked eye, it is impossibleto prevent occlusion of the microperforations either by the produce orby adhesive labels applied to the packages when microperforations areplaced across and along the entire film. Finally, the size and locationof the microperforations in the film also makes it impossible for thepackaging user to quickly inspect the films for consistency ofperforation size and number. These deficiencies have been roadblocks inthe wide spread commercialization of films made according to thisinvention.

As indicated, the current practices of producing microperforatedmaterials for modifying or controlling the atmosphere inside freshproduce packages are not satisfactory. There is a need for packaging inwhich the microperforations are registered in a small identifiable areathat will not be blocked by adhesive labels or adjacent packages duringpackage stacking or handling. The fresh produce should be placed in aproduct-specific package where the microperforation size, location, andnumber of microperforations are optimally selected to obtain the desiredfilm gas transmission rates and gas flux for maintaining the quality ofthat specific produce item. In addition, a method is needed foraccurately predicting the size and number of microperforations requiredby a particular weight of respiring produce at a specified temperatureto maintain a pre-selected atmosphere inside the package during storage.And, there needs to be a cost-effective method of manufacturingmicroperforated packaging materials according to the requirements of thepresent invention.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide aregistered microperforated polymeric packaging material with themicroperforations situated in well-defined target areas of the packagingmaterial.

Another object of the invention is to provide a means of calculating gastransmission requirements of respiring foods contained in registeredmicroperforated polymeric packaging materials with microperforationshaving specific size, shape, location, and number in order to optimizethe shelf life and quality of respiring foods.

A further object is to provide a packaging system, wherein registeredmicroperforated polymeric packaging materials wherein specific size,type and location of the microperforations is matched to specificcharacteristics of respiring fresh produce to optimize storage life.

Another object of the invention is the method of manufacturingregistered microperforated polymeric packaging materials, using a lasermounted above a stationary or moving polymer film web. The web-handlingequipment can be a bagmaking machine, a slitter/rewinder, a printingpress, a stand-alone web stopper, or a thermoforming unit. The system ofthe present invention employs a photoelectric sensor or other electricalmeans to signal the laser to ensure the microperforations are placed ina small identifiable area on the polymer web. There is a systemcontroller, such as a PLC (programmable logic controller), a PC(personal computer) or a combination of both, that takes the input fromthe sensor or other electrical signal and commands the laser to fire.The controller may also control the moving web.

This invention is directed to the specification, production, and use ofproduct-specific, registered microperforated polymeric packagingmaterials selected from the group consisting of polyethylene,polypropylene, polyester, nylon, polystyrene, styrene butadienecopolymers, cellophane, and polyvinyl chloride, in monolayers,coextrusions, or laminates, for extending the quality and shelf life ofrespiring foods, particularly fresh fruits, fresh vegetables, freshherbs, and fresh flowers, contained within the packaging.

Another object of the invention is a means of calculating the number ofmicroperforations of a preferred size, for example, 120 to 160 microns,specified for the polymer packaging material to maintain pre-selectedlevels of O₂, CO₂, and H₂O inside packages containing respiring freshproduce. The calculations can establish the optimal number ofmicroperforations required in the packaging material for eachmicroperforation size and shape.

And yet a further object of the invention is to provide a packagingsystem for the industry, wherein there is a matching of the packagingmaterial gas transmission rates and the respiration rates of the freshproduce to maintain pre-selected atmospheres inside the packages duringstorage. The packaging is optimized for a particular item, extending thefreshness and quality of the produce.

The present invention is an improved packaging for establishing optimalatmospheric conditions for respiring fresh fruits, vegetables, herbs andflowers, comprising a polymeric material with a set of microperforationsin the polymeric material to control the atmosphere within specified O₂and CO₂ concentrations in the presence of the respiring fresh produce,wherein the set of microperforations are placed in a registered targetarea on the polymeric material. The improved packaging material can beused to form a bag or a lidding film or a semi-rigid container.

The present invention is susceptible to many variations, including wherethe polymeric material is a heat-sealable film. Or where the polymericmaterial is formed into a semi-rigid container with a thickness in therange of 0.025 cm to 0.076 cm. And, where the polymeric material isselected from the group consisting of polyethylene, polypropylene,polyester, nylon, polystyrene, styrene butadiene, cellophane, andpolyvinyl chloride, their blends, coextrusions, and laminates.

The present invention also includes a means of calculating the total O₂Flux (cc O₂/day-atm) required by a particular product based on produceweight, respiration rate, storage temperature, and desired gascomposition inside the package. The total O₂ Flux of the package issatisfied by calculating the O₂ Flux provided by the breathable,non-perforated surface area of the packaging material and determiningthe size, shape, and number of microperforations required to meet thetotal O₂ Flux requirements of the package. In the preferred embodiment,the optimal size, shape and number of the set of microperforations forthe particular product is used for the registered target area. In mostcases, the target area is a small identifiable area in an upper third orquarter of the package. More preferably, the registeredmicroperforations are placed in any area that will not be occluded byproduce or other packages during shipping and storage.

Each of the microperforations has a preferred average diameter between110 and 400 microns, and more preferably 120–160 microns. It is furtherdesired that the polymeric material that is microperforated have a CO₂transmission rate that is 2.4 to 5.0 times greater than the film OTR,preferably 3.4 to 4 times greater than the film OTR.

The aspects of the present invention include a system of packaging freshproduce comprising the steps of calculating the total O₂ Flux requiredfor a given weight of respiring produce item, package surface area,storage temperature, and a pre-selected O₂ and CO₂ atmosphere. Next,determining an optimal packaging material with a desired CO₂/O₂transmission rates wherein the packaging material contains registeredmicroperforations designed for said O₂ Flux, placing the produce in thecontainer derived from the packaging material, and hermetically sealingsaid container.

A further object of the invention is a microperforated packaging for agiven quantity of respiring food produced by the process of calculatingthe number of microperforations for the given quantity of said respiringfood, locating a target area for the microperforations, positioning alaser over said target area, and drilling the microperforations in thetarget area.

Still another object is a microperforation system for makingmicroperforations in a target area of packaging material, comprising apolymeric web, having a laser mounted over the web, a sensor means toidentify the target area on the packaging material, and a means tocontrol the laser to drill the microperforations in the target area.Laser drilling software is used to increase efficiency.

The microperforation system can be used on a stationary (stopped) webwhere the laser beam moves over the packaging material to drill theholes. The laser system is interconnected to a two-axis beam scanner,which directs the laser beam to drill holes in the desired location.Alternatively, the microperforation system can consist of a stationarylaser beam and a moving polymer web. The laser is a CO₂ laser in thepreferred embodiment. In order to provide registration of perforations,a photoelectric sensor is used to find the eye mark on the polymericfilm or an electrical signal from the web-handling equipment is used tosignal the laser to fire at a preselected location on the film.

A basic intent of the present invention is to make a system forcomputing an optimal number and size of microperforations to control apackage atmosphere within specified O₂ and CO₂ concentrations. Thissystem also has a means of computing an optimal number ofmicroperforations to control package moisture vapor transmission rateswhile maintaining pre-selected O₂ and CO₂ concentrations.

An object of the invention is an improved packaging for establishingoptimum atmospheric conditions for respiring fresh fruits, vegetables,herbs and flowers, comprising a polymeric material, a set ofmicroperforations on the polymeric material, wherein the set ofmicroperforations are calculated to control the optimum atmosphericconditions within specified O₂ and CO₂ concentrations for the respiringfresh fruits, vegetables, herbs and flowers, and wherein the set ofmicroperforations are placed in a registered target area on thepolymeric material.

A further object is an improved packaging for establishing optimumatmospheric conditions for respiring fresh fruits, vegetables, herbs andflowers wherein the polymeric material is selected from the groupconsisting of polyethylene, polypropylene, polyester, nylon,polystyrene, styrene butadiene, cellophane, and polyvinyl chloride, inmonolayers, coextrusions, and laminates. Furthermore, an improvedpackaging material wherein the polymeric material is heat-sealable.

Other objects include an improved packaging material wherein thepolymeric material has a thickness in the range of 0.4 to 8 mil. Animproved packaging material wherein the polymeric material provides atotal O₂ Flux ranging from 150 cc/day-atm to 5,000,000 cc/day-atm. Animproved packaging material wherein the preferred polymeric materialprovides a total O₂ Flux ranging from 200 cc/day-atm to 1,500,000cc/day-atm.

And yet another object of the invention is an improved packagingmaterial wherein the polymeric material forms a bag. Also, an improvedpackaging material wherein the polymeric material is a heat sealablelidding film. An improved packaging material wherein the polymericmaterial is formed into a semi-rigid container with a thickness greaterthan 8 mil.

An object of the invention is an improved packaging material furthercomprising a means of calculating an optimal number of the set ofmicroperforations in the registered target area. An improved packagingmaterial further comprising a means of calculating an optimal size ofthe set of registered microperforations. Also including an improvedpackaging material wherein the registered target area is a smallidentifiable area in an upper quarter of the package. Further objectsinclude an improved packaging material wherein the registered targetarea is a small identifiable area in an upper third of the package. Animproved packaging material wherein the registered target area islocated in an area that prevents occlusion of the microperforations byproduct or other packages. Additionally, an improved packaging materialwherein each of the microperforations has an average diameter between110 and 400 microns, preferably 120–160 microns. Finally, an improvedpackaging material wherein the polymeric base material has a CO₂transmission rate that is 2.5 to 5.0 times greater than the O₂transmission rate, most preferably 3.4 to 4.0 times greater.

Yet a further object is a system of packaging fresh produce comprisingthe steps of calculating the total oxygen flux of the polymeric materialrequired for a given weight of respiring produce item, package surfacearea, storage temperature, and a pre-selected O₂ and CO₂ atmosphere,determining an optimal packaging material, wherein the packagingmaterial contains registered microperforations designed for the O₂ andCO₂ transmission, placing the produce in the container derived from thepackaging material and closing the container.

An object of the invention is a process of producing a microperforatedpackaging system for a given quantity of respiring food comprising thesteps of selecting a polymeric packaging material for optimal O₂ and CO₂transmission rates, calculating a number of microperforations requiredin the packaging material for the given quantity of the respiring food,locating a target area for the microperforations, positioning a laserover the target area, and drilling the microperforations in the targetarea.

An object includes a microperforation system for makingmicroperforations in a registered target area of packaging material,comprising, a polymeric web, a laser mounted over the web, a sensormeans to identify the target area on the packaging material, a means tocontrol the laser to drill the microperforations in the target area.Additionally, a microperforation system wherein the laser is a CO₂laser. Also, a microperforation system wherein the sensor is selectedfrom the group comprising a through-beam photoelectric sensor and aphotoelectric proximity sensor.

And yet another object is a microperforation system wherein a controllerdirects the laser to drill holes in the target area with the target areaidentified using an electrical signal from the web-handling equipment.And, a microperforation system wherein the web is moving and the laseris stationary. It being understood that the term laser refers to thelaser beams emanating from the laser delivery head or similar deliverydevice, and the laser optical system provides a focused laser beam.

And another object of the invention is a microperforation system furthercomprising a means of computing an optimal number and size ofmicroperforations to control a package atmosphere within specified O₂and CO₂ concentrations. Also, a microperforation system furthercomprising a means of computing an optimal number of microperforationsto control package moisture vapor transmission rates while maintainingpre-selected O₂ and CO₂ concentrations. A microperforation systemwherein the web is stationary and the laser is moving. And also, amicroperforation system wherein the laser comprises a two-axis beamscanner mounted over the web.

A final object is a produce packaging material produced by the processcomprising the steps of selecting an appropriate polymeric base materialfor specified CO₂/O₂ transmission rates and the quantity of the produce,calculating an optimal number/size of microperforations, locating atarget area, positioning a laser over the target area, and drilling themicroperforations in the target area with the laser.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein only a preferred embodiment of theinvention is described, simply by way of illustration of the best modecontemplated for carrying out the invention. As will be realized, theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects, allwithout departing from the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1 depicts a microperforation system of a stationary film webwherein the moving laser beam drills the microperforations in the targetarea of what will be a finished bag.

FIG. 2 shows a bag produced by the stationary web microperforationprocess.

FIG. 3 depicts a microperforation system wherein the stationary laserbeam drills the microperforations in the target area of the film, as theweb is moving.

FIG. 4 shows a bag produced by the moving web microperforation process.

FIG. 5 shows a Type I microperforation with an aspect ratio of 1 to 1.2,and shows Type II microperforations with an aspect ratio>1.2.

FIG. 6 depicts the oxygen and carbon dioxide contents inside 3 lb.packages of broccoli florets sealed in registered microperforated bagshaving 36, 150-micron perforations. Storage temperature was 4–5° C.

FIG. 7 depicts carbon dioxide content inside 3 lb. packages of broccoliflorets sealed in registered microperforated bags having 36, 150-micronperforations with base packaging films having different carbon dioxidetransmission rates. Storage temperature was ₄–5° C.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description, the units applied to terms used inreference to the O₂, and CO₂ transmission rates of a packaging material,i.e., “OTR and CO₂TR”, respectively, are expressed ascc/m²-day-atmosphere at 25° C., 75% RH. In the pressure units, oneatmosphere (atm) is 101,325 kg/ms². The units describing the flow of aparticular gas through a packaging material are “flux”, expressed ascc/day-atm. The units applied to moisture vapor transmission (“MVTR”) ofa film are expressed as g H₂O/m²-day-atm at 25° C., 75% RH.

As shown in FIG. 1, a CO₂ laser drilling system can be used to drillmicroperforations in a stopped polymer web (hereafter called “stoppedweb method”). This is essentially a low speed (50–150 ft/min) method formicroperforating packaging materials using lasers mounted over theweb-handling equipment. A controller 20 operates with input from thethrough-beam photoelectric sensor 30 to position the polymer filmsheeting or tubing 40 and locate the target area 50. The polymer film 40has an eye mark 60 that is detectable by the sensor 30. Once the eyemark 60 is detected, the registration of the film 40 is determined andthe controller 20 sends a signal to the laser/optics power supply 70which directs the laser beam to the scan head 80 where the laser beam 90is directed onto the film 40 creating microperforations 100 in apre-selected array defined by a computer program. The registeredmicroperforated film 40 is then converted into bags using bagmakingmachines known to the industry. As an example, a polymer bag 120 withside seals is shown in FIG. 2, and the microperforations 100 are in aspecific target area 50 of the bag 120.

For example, a simple drilling system consists of a 10-watt sealed beamair-cooled CO₂ laser with a controller 20, a power supply and focusingoptics 70, and a two-axis beam scanner 80 mounted horizontally above thepackaging material 40 web on a bagmaking machine, a slitter/rewinder, aprint station or a stand-alone web stopper. The horizontal position ofthe laser system mounted over the web can be adjusted by mechanical orelectronic methods so that the laser beam 90 is positioned over the areaof the web to be microperforated 50. The laser system 70 is linked to acontroller 20 that is also linked to a photosensor 30 or a mechanicaltimer on the web-handling equipment. In a preferred embodiment, allcomponents are interfaced to a computer that uses laser drillingsoftware to direct the laser to microperforate the packaging material ina pre-selected array.

Registered microperforations are produced by linking the laser drillingprocess to a signal from a photosensor 30 mounted on a bagmakingmachine, a web stopper, a slitter/rewinder, or a printing press. Whenthe photosensor 30 detects a printed eye mark 60 on the packagingmaterial 40, this signal is used to direct the controller 20 to fire thelaser 70 to drill a specified number of perforations in the packagingmaterial in a pre-determined array, e.g. in 1 cm² with 150-micronperforations at each corner of the square, at a specific location on theweb. Microperforation arrays 100 are normally positioned near what willbecome the upper one-quarter or one-third of the bag, as shown in FIG.2, so when filled packages are placed in case cartons they are notoccluded by adjacent packages in the carton. For heat-sealable liddingfilms used with rigid plastic trays, the perforations 100 are registeredin areas of the lid that will not be occluded during stacking.

The photosensor detection process uses a printed eye mark on thepackaging material. Photoelectric sensors detect an object when itinterrupts the sensing path. For example, the edge of a film is printedwith a colored eye mark (a small, solid rectangular bar, usually blackin color) at defined locations along the web to designate wheremicroperforations are to be drilled in the film. A photoelectric sensoris mounted on the web-handling equipment. When the sensor beam passesover the eye mark on the film, the eye mark interrupts the beam andtherefore, the photoelectric sensor senses it. This beam interruption isdetected by the controller that then directs the laser to drill aspecified number of perforations in the film directly across from theeye mark or at a defined distance from the eyemark in a location wherethe laser head is positioned above the plastic web. The location of themicroperforations from the eye mark can be varied by moving the laserhousing, either mechanically or electrically, over the target area ofthe plastic web.

For other photoelectric sensing modes, it is not necessary to have aprinted eye mark to detect the object. In the photoelectric proximitymode, an object is sensed when the sensor's own transmitted energy isreflected back from the object's surface. If there is no object present,this reflection does not occur. Proximity sensors could be used tosignal the laser to microperforate semi-rigid plastic trays for freshproduce packaging.

A wide range of polymer materials (monowebs, coextrusions, andlaminates), can be microperforated with lasers, including polypropylene,polyethylene, polyester, polystyrene, styrene butadiene copolymers,nylon, cellophane, or polyvinyl chloride. The preferred drilling methoduses CO₂ lasers mounted over stationary or moving polymer webs.Photoelectric sensor methods or other electrical signals can be used toregister microperforations in polymer materials. The photoelectricsensor method is accurate and reliable and is the preferred method ofthis invention.

When the stopped web method is used on a bagmaking machine, the drillingcan occur during the heat-sealing portion of the bagmaking cycle,because in this short time span (about 400 msec), the web is essentiallystationary. In effect, the web is stopped; an electrical signal from thebagmaking machine timer directs the laser to commence the drilling.Alternatively, a stand-alone web stopper can be used to stop the web(using a series of accumulator rolls) for the drilling operation.

From 1 to over 200 microperforations can be drilled into the packagingmaterial during a single stop phase (stopped web method) with the beamscanner perforating in a serpentine pattern to minimize totalperforation time. The more holes that are drilled, the longer thedrilling operation takes. For most applications 1–100 microperforationsare needed with 1–7 msec drilling time for each microperforation. Ifdrilling occurs through two thickness of material at the same time, aswould be the case when microperforating plastic tubing, then the numberof holes drilled per laser firing doubles.

For the stopped web method that uses a two-axis beam scanner, themicroperforations can be drilled in a variety of different patterns orarrays, e.g., straight lines, rectangles, squares, and circles. The mosttime-efficient method is to place the microperforations in a straightline or square. If microperforations are placed in a square or arectangular array, the most time-efficient drilling occurs when thelaser follows a serpentine pattern. The size of the microperforations isdetermined by adjusting the laser power (30–100% of the maximum power)and drilling duration. Higher power and longer duration give largermicroperforations than lower power and shorter duration. Preferably, thelaser should be set at 70% of maximum power and the duration should bevaried to produce the desired perforation size. As the perforation sizeincreases the OTR of the perforation also increases.

Registered microperforations 100 can be drilled in moving packagingmaterial webs, as shown in FIG. 3, using higher power CO₂ lasers.Hereafter this process will be referred to as “microperforating on thefly.” A controller 20 for the moving web receives input from thephotoelectric sensor 30, and controls the power supply 200 to the laseroptics. As the polymer film 40 moves through the web-handling equipment,the sensor 30 detects the eye mark 60, and the signal is communicated tothe controller 20. The controller 20 operates the laser power supply andoptics 200, which, in turn, powers the laser and directs the laser beamto the stationary laser delivery head 210 to drill a specific number ofmicroperforations 100 in the target area 50. As an example, a polymerbag 120 with side seals is shown in FIG. 4, and the microperforations100 are in the specific target area 50 of the bag 120.

The power requirements (25-watt to >700-watt) depend on the speed theweb will be traveling, and the composition and the thickness of thematerials to be drilled. Faster speeds and thicker packaging materialsrequire higher power lasers than slower speeds and thinner packagingmaterials. For microperforating on the fly, a stationary laser beamdelivery head mounted on a printing press, slitter/rewinder, orbagmaking machine is used to produce a specified number ofmicroperforations, usually one to 50, in a short line, generally 7 cm orless, running in the machine direction of the moving film web. When thephotosensor 30 detects a printed eye mark on the packaging material,this signal is used to trigger the laser to drill a specified number ofperforations 100 in a straight line on the material in the target area50 of the impression. In this application, the laser power and the speedof the web determine the size and shape of the microperforations thatare made. The faster the speed, the more elongated the microperforationsbecome. If multiple segments of the same bag or package impression(boundaries of the package) must be microperforated to provide thenecessary O₂ Flux, then multiple eye marks 60 are needed to signal thelaser 200 to fire at each location in the bag or package impression.

If more than one lane of microperforations is needed formicroperforating on the fly, as would be the case for microperforatingtwo side-by-side printed impressions of a packaging material or twomicroperforation lanes in the same impression, multiple lasers can bemounted on the web handling equipment or a beam splitter can be used tosplit the beam from one laser to multiple delivery heads.

Various hole sizes and shapes are created when plastic materials areperforated with lasers. Whatever laser drilling method is used, eitherstopped web or microperforating on the fly, the packaging materialcomposition, degree of orientation, and structure (monolayer,coextrusion, or laminate) affect the size, shape and O₂ Flux of theresulting microperforations. Computer software that directs the laserperforating process can affect the shape of the perforation. However,software factors can be easily changed to alter the perforation shape.In contrast, polymer materials have inherent physical/chemicalcharacteristics that will impact the hole size and shape for any givenpower and pulse duration. For example, stationary monowebs ofpolyethylene films perforated with a beam from a 10-watt CO₂ laser(stopped web), produce perforations that are elongated, having aspectratios (ratio of the longest to the shortest diameter) greater than 1.2.In contrast, when heat-seal coated polyester films are perforated underidentical conditions, the perforations are nearly spherical with aspectratios of 1–1.2. When lasers drill moving polymer webs, faster speeds(>300 ft/min) may produce more elongated perforations, with aspectratios of 1.8, depending on the polymer film composition and speed ofthe web. These differences in microperforation shape affect the O₂ Flux.Perforations with the same size long diameter but different size shortdiameters have different O₂ Flux. That is, microperforations with aspectratios (ratio of the longest to the shortest diameter) of close to 1have higher O₂ Flux than perforations with aspect ratios >1.

A wide range of microperforation sizes can be used to control theatmosphere inside fresh produce packaging. However, in the preferredembodiment, microperforations in the range of 110–400 microns (longestdiameter), preferably, 120–160 microns, offer the most benefits incontrolling desired O₂ and CO₂ levels inside the package. Based on ourresearch with a wide range of polymer materials, microperforation shapescan be classified into two broad categories. Type I category consists ofholes with an aspect ratio (the ratio of the longest diameter to theshortest diameter) of 1–1.2, as shown in FIG. 5. This is typical forpolyesters and polypropylene films with heat-seal coatings.

Type II category of microperforations, observed in polyethylene monowebsand polyethylene coextrusions, has an aspect ratio >1.2, illustrated inFIG. 5. There is an elongation (slight to exaggerated) of themicroperforation, often in the direction of the film orientation,forming an oval or elliptical shape.

An analytical method using an oxygen sensor was used to determine the O₂Flux through microperforations of individual, 150-micron (longestdiameter) perforations from the two categories of microperforationsdescribe above. The average values for O₂ Flux for one, 150 micronperforation are as follows:

Type I microperforation (aspect ratio 1.0–1.2) is 250 cc/day-atm.

Type II microperforation (aspect ratio>1.2) is 200 cc/day-atm.

Experimental results showed that the range of O₂ Flux values for thesemicroperforations varies by +/−6% of the stated value. The O₂ Flux ofthe microperforations is not dependent on the thickness of the film thatis microperforated.

The range of O₂ Fluxes that can be created by registeringmicroperforations, in polymer materials by the laser methods describedabove, is very broad. Although microperforated films, according to thepresent invention, can be made with an O₂ Flux ranging from 150cc/day-atm to over 5,000,000 cc/day-atm, the preferred range is 200 to1,500,000 cc/day-atm for controlling or modifying the atmosphere insidefresh produce packages varying in weights from 19 g to several thousandkg.

Knowing the O₂ Flux of individual microperforations makes it arelatively simple task to calculate the size and number ofmicroperforations needed to establish a desired atmosphere inside apackage containing fresh fruit, fresh vegetables, fresh herbs, freshflowers or other biochemically active foods.

EXAMPLE 1

To determine the number and size of microperforations needed for eachproduce item in a particular package, the gas transmission requirements(O₂, CO₂, moisture vapor transmission rate—MVTR) for the total packageis first determined. The contribution the microperforations must make tothe total gas transmission properties of the package is affected by:

-   -   produce respiration rate,    -   weight of produce to be packaged,    -   desired atmosphere in the package,    -   breathable surface area of the package,    -   gas transmission properties (O₂, CO₂, MVTR) of the packaging        material to be microperforated, and    -   expected storage temperatures during the life of the product.

Different fresh produce items, whether whole or fresh-cut, havedifferent respiration rates. Cutting the produce item generallyincreases the respiration rate by 2-fold or more.

Equations (1), (2), and (3) below can be used to determine the total O₂Flux (Flux_(O2-Total)) requirements of a fresh produce package,including the O₂ Flux of the breathable area of the packaging film(Flux_(O2-film)), and the O₂ Flux of the microperforations(Flux_(O2-MP)), required to maintain a desired atmosphere inside apackage containing a specific fresh fruit, fresh vegetable, fresh herbor fresh flower.OTR _(T) [(M×RR)/(A _(S) P(0.21−IntO ₂))]×24 hrs/day  (1)where,

OTR_(T) = total OTR required for the package in cc O₂/m²-day-atm M =mass of produce (kg) RR = respiration rate (cc O₂/kg/hr) @ the expectedstorage temperature A_(s) = breathable surface area of the package (m²)P = atmospheric pressure (atm), assumed to be 1 Int O₂ = desired O₂atmosphere inside the package stated as a volume fraction (i.e., if thedesired O₂ is 8%, the volume fraction is 0.08).

The value 0.21 represents the volume fraction of O₂ in ambient air.

For example: To establish an atmosphere at 5° C. of 8% O₂ and 10–15% CO₂inside a 25.4 cm wide×40.6 cm long×50 micron thick polyethylene bag(OTR_(base-film)=3100 cc/m²-day-atm) containing 1.36 kg of broccoliflorets, with an O₂ respiration rate of 31 cc/kg/hr, the OTR_(T)required by the package would be:

$\begin{matrix}{{OTR}_{T} = \left\lbrack {{\left( {1.36\mspace{14mu}{kg} \times 31\mspace{11mu}{cc}\text{/}{kg}\text{/}{hr}} \right)/\;\left( {2\left( {0.254\mspace{14mu} m \times 0.356\mspace{14mu} m} \right)} \right)} \times} \right.} \\{\left. {1\mspace{14mu}{atm}\mspace{14mu}\left( {0.21 - 0.08} \right)} \right\rbrack \times 24\mspace{14mu}{hr}\text{/}{day}} \\{= {43\text{,}038\mspace{14mu}{cc}\text{/}m^{2}\text{-}{day}\text{-}{atm}}}\end{matrix}$Note that the breathable surface area is less than the total dimensionsof the bag because the top seal and the film skirt beyond the seal aresubtracted since they do not contribute to the OTR_(T).

Once the OTR_(T) requirements for a particular item and package size aredetermined from equation (1), then the O₂ flow through the breathablesurface area of the bag per day (Flux_(O2-film) in cc/day-atm), iscalculated using equation (2):Flux_(O2-film) (cc/day-atm)=OTR _(base-film)(cc/m²-day-atm)×A_(S)(m²)  (2)For example: The dimensions of the breathable area of a plastic bag usedto package 1.36 kg of fresh-cut broccoli are 25.4 cm×35.6 cm (×2 for 2sides), and the OTR of the base film is 3100 cc/m²-day-atmosphere. TheFlux_(O2-film) (cc/day-atm) through the breathable area of the bag is:

$\begin{matrix}{{{{Flux}_{02\text{-}{film}}\left( {{cc}\text{/}{day}\text{-}{atm}} \right)} = {\left( {3100\mspace{14mu}{cc}\text{/}m^{2}\text{-}{day}\text{-}{atm}} \right) \times 0.181\mspace{14mu} m^{2}}}\;} \\{= {561\mspace{14mu}{cc}\text{/}{day}\text{-}{atm}}}\end{matrix}$However, a total Flux_(O2-film) Total of 7790 cc/day-atm is needed forthis package:Flux_(O2-total) =OTR _(T) cc/m²-day-atm×A_(S)(m²)Flux_(O2-total)=43,038 cc/m²-day-atm×0.181 m²=7790 cc/day-atmTherefore, the majority of Flux_(O2-Total) must be supplied by themicroperforations (Flux_(O2-MP)):

$\begin{matrix}\begin{matrix}{{Flux}_{02\text{-}{MP}} = {{Flux}_{02\text{-}{Total}} - {Flux}_{02\text{-}{film}}}} \\{= {{7790\mspace{14mu}{cc}\text{/}{day}\text{-}{atm}} - {561\mspace{14mu}{cc}\text{/}{day}\text{-}{atm}}}} \\{= {7229\mspace{14mu}{cc}\text{/}{day}{\text{-}\text{atm}}}}\end{matrix} & (3)\end{matrix}$The number of 150 micron (longest diameter) perforations required inthis 2 mil polyethylene package is equal to:(7229 cc/day-atm)/(200 cc/day-atm per Type II microperforation)=36

For this film, if the longest diameter of the average microperforationis smaller or larger than 150 microns, the number of perforations can beadjusted to meet the required Flux_(O2-total) for the package. Forexample, if the microperforations made in the polyethylene film were 120microns (Type II) in the longest diameter, then each 120 micronperforation would have a Flux_(O2-120μ) of 160 cc/day-atm:Flux_(O2-120)μ, perf=(120 micron×200 cc/day-atm)/(150 microns)=160cc/day-atm

To maintain an 8% O₂ and 10–15% CO₂ atmosphere inside a 0.181 m²polyethylene bag containing 1.36 kg broccoli florets stored at 5° C., itwould require 45, 120-micron perforations to give the same Flux_(O2-MP)as 36, 150-micron perforations.

To test the accuracy of the method to predict the size and number ofmicroperforations required to maintain a desired atmosphere inside apackage containing a respiring produce item, 50 micron polyethylenetubing was blown by standard extrusion methods and used to make 25.4 cmwide×40.6 cm long bags which were microperforated in-line on a bagmakingmachine using a 10-watt CO₂ laser and the stopped web method.Thirty-six, 150 micron perforations were registered in a 6.45 cm² (1in²) array located 7.6 cm from the open end of the bag and 5 cm from theside seal. An electrical signal from the bagmaking machine was used totrigger the laser to fire at the same time the heat-sealing bar made thebag side seal.

Broccoli florets were prepared at 4° C. in a commercial processing plantand 1.36 kg was packaged into each bag (25.4 cm wide×40.6 cm long, 50micron thick). Filled bags were packed vertically into corrugatedcartons with the registered microperforations at the top of the carton.Cartons were stored at 4–5° C. for 14 days. FIG. 6 represents the %headspace gas concentration over time, in microperforated bags (36microperforations with an average size of 150 microns) containing 1.36kg of broccoli florets. A steady state atmosphere of 8–10% O₂ and 8–12%CO₂ was reached after 60 hrs at 5° C. After 2 weeks at 5° C., floretcolor remained bright green, there was little or no evidence of browningat the cut ends, and no off-odors were observed on opening the bag.

EXAMPLE 2

Equations (1), (2), and (3) were used to determine the size and numberof microperforations needed to maintain an atmosphere of 10–12% O₂ and8–10% CO₂ inside a polyethylene pallet bag (125 cm wide×102 cm fullgusset×203 cm long×100 micron thick) containing 217.9 kg of fresh sweetcherries at 1.1 C. The OTR_(base)=1085 cc/m²-day-atm. Ninety-one cm ofthe bag length will be used to seal the bag by gathering the plastic atthe top of the pallet, twisting it, doubling over the neck, and closedtightly with an electrical tie. Therefore, the breathable area of thebag will be 125 cm wide×102 cm gusset×112 cm long.

The equations for this package are:

$\begin{matrix}\begin{matrix}{{{OTR}_{T}\left( {{cc}\text{/}m^{2}\text{-}{hr}\text{-}{atm}} \right)} = {\left( {217.9\mspace{14mu}{kg} \times 5\mspace{14mu}{cc}\text{/}{kg}\text{/}{hr}} \right)\text{/}}} \\{\left( {2.54m^{2} \times 1\mspace{14mu}{atm}\mspace{14mu}\left( {0.21 - 0.11} \right)} \right.} \\{{{OTR}_{T}\left( {{cc}\text{/}m^{2}\text{-}{d{ay}}\text{-}{atm}} \right)} = {4289\mspace{14mu}{cc}\text{/}m^{2}\text{-}{hr}\text{-}{atm} \times 24\mspace{14mu}{hr}\text{/}{day}}} \\{= {102\text{,}936\mspace{14mu}{cc}\text{/}m^{2}\text{-}d\;{ay}\text{-}{{atm}.}}}\end{matrix} & (1) \\\begin{matrix}{{{Flux}_{02\text{-}{film}}\left( {{cc}\text{/}{day}\text{-}{atm}} \right)} = {1085\mspace{14mu}{cc}\text{/}m^{2}\text{-}{d{ay}}\text{-}{atm} \times 2.54m^{2}}} \\{= {2756\mspace{14mu}{cc}\text{/}{day}\text{-}{atm}}}\end{matrix} & (2) \\\begin{matrix}{{Flux}_{02\text{-}{MP}} = {\left( {102\text{,}936\mspace{14mu}{cc}\text{/}m^{2}\text{-}{d{ay}}\text{-}{atm} \times 2.54m^{2}} \right) -}} \\{2756\mspace{14mu}{cc}\text{/}{day}\text{-}{atm}} \\{= {258\text{,}701\mspace{14mu}{cc}\text{/}{day}\text{-}{atm}}}\end{matrix} & (3)\end{matrix}$

Microperforations were drilled into a 58 cm² area 86 cm up from thebottom seal and in the middle of the front panel of the bag (125 cmwide×102 cm gusset×203 cm long, 100 micron thick). Both sides of the bagwere microperforated with a laser using the stopped web method. Thetotal number of 300-micron microperforations drilled into each bag was646 (323 in the front panel and 323 in the back panel):(258,701 cc/day-atm)/(400 cc/day-atm for each 300 micron Type IImicroperforation)=647

Twenty-four boxes of cold Bing cherries, each containing 9.1 kg ofcherries, were stacked inside the microperforated pallet bag that wasdraped over a 102 cm×122 cm wooden pallet. The filled bag was pulledover the top of the cartons, it was hermetically sealed, and the palletwas stored at 1.1 C. After 8 weeks at 1.1 C, headspace gas analysis ofthe sealed pallet bag indicated that the cherries had established anatmosphere of 12% O₂ and 8% CO₂. Sensory evaluation of the cherriesshowed that flesh color was maintained, cherry stems were green and hadnot darkened during storage, there was no evidence of mold growth, andeating quality was good.

EXAMPLE 3

Equations (1), (2) and (3) were used to determine the size and number ofmicroperforations needed to maintain an atmosphere of 10% O₂ and 10–15%CO₂ (at 5° C.) inside a 15-cm diameter semi-rigid bowl (0.056 cm thickpolyester/PE laminate) containing 227 g fresh-cut cantaloupe and sealedwith a flexible heat-sealable lidding film made from a laminate oforiented polypropylene and polyethylene (OTR_(base)=1550 cc/m²-day-atm).The OTR of the bowl does not contribute significantly to theFlux_(O2-Total).

The equations for this package are:

$\begin{matrix}\begin{matrix}{{{OTR}_{T}\left( {{cc}\text{/}m^{2}\text{-}{hr}\text{-}{atm}} \right)} = {\left( {0.23\mspace{14mu}{kg} \times 6\mspace{14mu}{cc}\text{/}{kg}\text{/}{hr}} \right)/}} \\{\left( {1.8 \times 10^{- 2}m^{2} \times 1\mspace{14mu}{{atm}\left( {0.21 - {.10}} \right)}} \right)} \\{{{OTR}_{T}\left( {{cc}\text{/}m^{2}\text{-}{d{ay}}\text{-}{atm}} \right)} = {697\mspace{14mu}{cc}\text{/}m^{2}\text{-}{hr}\text{-}{atm} \times 24\mspace{14mu}{hr}\text{/}{day}}} \\{= {16\text{,}728\mspace{14mu} c\; c\text{/}m^{2}\text{-}{day}\text{-}{{atm}.}}}\end{matrix} & (1) \\\begin{matrix}{{{Flux}_{02\text{-}{film}}\left( {{cc}\text{/}{day}\text{-}{atm}} \right)} = {1550\mspace{14mu}{cc}\text{/}m^{2}\text{-}d{ay}\text{-}{atm} \times 1.8 \times 10^{- 2}m^{2}}} \\{= {28{\mspace{11mu}\;}{cc}\text{/}{day}\text{-}{atm}}}\end{matrix} & (2) \\\begin{matrix}{{Flux}_{02\text{-}{MP}} = {\left( {16\text{,}728\mspace{14mu} c\; c\text{/}m^{2}\text{-}{day}\text{-}{atm} \times 1.8 \times 10^{- 2}m^{2}} \right) -}} \\{28\mspace{14mu}{cc}\text{/}{day}\text{-}{atm}} \\{= {273\mspace{14mu}{cc}\text{/}{day}\text{-}{atm}}}\end{matrix} & (3)\end{matrix}$One, Type II 205-micron perforation (produced with a stationary laserbeam and a moving polypropylene/polyethylene laminate film web) isneeded to provide the necessary OTR for 227 g cantaloupe stored in asemi-rigid tray at 5° C.

Samples of polypropylene/polyethylene lidding film were microperforatedusing a stationary laser and a moving polymer film web (microperforatingon the fly). One microperforation, with an average diameter of 200–210micron, was drilled into each impression at the eye mark. Themicroperforated lidding film was used to seal fresh-cut cantaloupeinside the 15 cm diameter bowls. Bowls were stored at 5° C. for 10 daysand evaluated for headspace gas contents and changes in quality.

Headspace analysis showed that, one, 200 to 210-micron perforationcontrolled the atmosphere inside 227 g packages of fresh-cut cantaloupeat desired O₂ and CO₂ levels and maintained satisfactory product qualityfor 10 days at 5° C. Having at least one microperforation in the packagealso prevented the package seals from rupturing due to changes inpressure when such packages were shipped over mountains in refrigeratedtrucks.

EXAMPLE 4

The moisture vapor transmission rate (MVTR) of a packaging material isalso important in maintaining the quality of packaged produce. Therelative humidity inside most produce packages is between 96 and 99%.High relative humidity, in combination with excess free water in thepackage, can limit fresh produce shelf life by fostering microbialgrowth that leads to watery/slimy deterioration of plant tissues. Mostnon-perforated polyethylene or polypropylene films have a MVTR of lessthan 15 g/m²-day. Microperforating the package can increase the MVTRrange from 30 to >600 g/m²-day. The increase in film MVTR caused bymicroperforations improves shelf life of water sensitive produce.

Packaged spinach deteriorates quickly if excess moisture accumulates inthe package during storage. Therefore, the most important goal inspinach packaging is to use microperforations to increase the packageMVTR without causing the leaves to dehydrate and lose their turgor. O₂levels should be held just below ambient air (i.e., O₂=16–19%) and CO₂levels slightly elevated above ambient air (CO₂=2–5%).

Equations (1), (2), and (3), were used to determine the size and numberof microperforations needed to maintain an atmosphere of 17–19% O₂ and2–3% CO₂ inside bags containing 284 g spinach and stored at 5° C. Bagssize was 25.4 cm wide×40.6 cm long (breathable area=25.4 cm×35.6 cm×2sides) and film composition was 76-micron polyethylene (OTR_(base)3255cc/m²-day-atm):

$\begin{matrix}\begin{matrix}{{{OTR}_{T}\left( {{cc}\text{/}m^{2}\text{-}{hr}\text{-}{atm}} \right)} = {\left( {0.284\mspace{14mu}{kg} \times 46\mspace{14mu}{cc}\text{/}{kg}\text{/}{hr}} \right)/}} \\{\left( {0.181\; m^{2} \times 1\mspace{14mu}{{atm}\left( {0.21 - 0.17} \right)}} \right)} \\{= {1804\mspace{14mu}{cc}\text{/}m^{2}\text{-}{hr}\text{-}{atm}}} \\{{{OTR}_{T}\left( {{cc}\text{/}m^{2}\text{-}{day}\text{-}{atm}} \right)} = {1804\mspace{14mu}{cc}\text{/}m^{2}\text{-}{hr}\text{-}{atm} \times 24\mspace{11mu}{hr}\text{/}{day}}} \\{= {43\text{,}296\mspace{14mu}{cc}\text{/}m^{2}\text{-}{day}\text{-}{atm}}}\end{matrix} & (1) \\\begin{matrix}{{{Flux}_{02\text{-}{film}}\left( {{cc}\text{/}{day}\text{-}{atm}} \right)} = {3255\mspace{14mu}{cc}\text{/}m^{2}\text{-}{day}\text{-}{atm} \times 0.181\mspace{14mu} m^{2}}} \\{= {589\mspace{14mu}{cc}\text{/}{day}\text{-}{atm}}}\end{matrix} & (2) \\\begin{matrix}{{Flux}_{02\text{-}{MP}} = {\left( {43\text{,}296{\mspace{11mu}\;}{cc}\text{/}m^{2}\text{-}{day}\text{-}{atm} \times 0.181\mspace{14mu} m^{2}} \right) -}} \\{589{\mspace{11mu}\;}{cc}\text{/}{day}\text{-}{atm}} \\{= {7248{\mspace{11mu}\;}{cc}\text{/}{day}\text{-}{atm}}}\end{matrix} & (3)\end{matrix}$Lasers make Type II microperforations in polyethylene. Therefore, this284 g bag requires 36, 150-micron perforations, i.e., (7248cc/day-atm)/(200 cc/day-atm per 150-μ perforation).

Thirty-six laser microperforations, with an average diameter of 150microns, were registered in a 6.45 cm² area, 7.6 cm from the bag openend and 7.6 cm from the side seal of the 25.4 cm wide×40.6 cm long bagsusing the stopped web method. Bags were filled with freshly washed andspun-dried spinach in a commercial 4° C. processing room beforeheat-sealing and storing at 5° C. for 14 days. After 3 days, theheadspace O₂ and CO₂ ranged from 17–20% and 1–3%, respectively, whichwas maintained throughout the 14-day storage study. Spinach leavesmaintained their bright green color and turgor and did not show signs ofdehydration or watery deterioration. Water loss from the package wasabout 2–3% after 14 days at 5° C.

EXAMPLE 5

The OTR and MVTR of packaging material are not the only important gastransmission rates in maintaining produce quality in MAP/CAP packages.CO₂TR determines the internal CO₂ atmosphere in the package and alsoaffects package appearance. A low CO₂TR and a low CO₂/O₂ ratio film maycause package puffing (distention). If CO₂TR is too high, a collapsedpackage may result. The CO₂/O₂ ratio of microperforations is 1. However,the following example shows the importance of selecting base polymerfilms for microperforating that have sufficient CO₂TR to maintain anacceptable package appearance and internal CO₂ concentration.

Three polymer films with similar base film OTRs and different CO₂TRswere microperforated by the stopped web method, registering 36microperforations in a 6.45 cc² area in the top quarter of the formedbag. Fresh-cut broccoli florets (1.36 kg) were sealed in the bags andstored at 5° C. As shown in FIG. 7, the O₂ atmosphere inside packagesmade from the different polymers equilibrated at 8–10% after 48 hrs andmaintained that level throughout storage. However, the content of CO₂inside the packages varied with the CO₂TR of the film. Packages with thelowest CO₂TR became distended during storage, and those with the highestCO₂TR collapsed, looking more like a vacuum package than a pillow pack.The base polymer film with a CO₂TR of 13,950 cc/m²-day-atm and aCO₂TR/OTR of 3.6 to 4.0 was optimum for preventing discoloration ofbroccoli cut ends and off-odors and for maintaining an acceptablepackage appearance.

EXAMPLE 6

Polymeric packaging materials used to make semi-rigid containers, rangein thickness from 0.025 cm to 0.076 cm. Delicate fruits likestrawberries, raspberries, and blueberries are routinely packaged incontainers having two semi-rigid parts: a rigid tray and a rigid lid. Wetested the hypothesis that a semi-rigid container with registeredmicroperforations in the semi-rigid lid could be used to control theatmosphere within a fresh produce package, thereby extending shelf life.

Polyvinyl chloride sheeting was thermoformed into a tray (0.143 mwide×0.197 m long×0.057 m deep, 0.056 cm thick) and rigid lid (0.143 mwide×0.197 m long×0.013 m deep) for packaging 0.454 kg slicedstrawberries. This 0.056 cm thick package is essentially impermeable,having an OTR of <7 cc/m²-day-atm. Therefore, all the O₂ Flux formaintaining a desired atmosphere of 8 to 10% O₂ and 10 to 15% CO₂ at 4°C. must come from the microperforations. The microperforations will beplaced in a 1 cm² (0.0001 m²⁾ area on the lid, essentially the onlybreathable portion of the package. Equations (1) and (3) were used todetermine the size and number of microperforations needed in the lid tomaintain the desired atmosphere inside the package:

$\begin{matrix}\begin{matrix}{{{OTR}_{T}\left( {{cc}\text{/}m^{2}\text{-}{hr}\text{-}{atm}} \right)} = {\left( {0.454\mspace{14mu}{kg} \times 22\mspace{14mu}{cc}\text{/}{kg}\text{/}{hr}} \right)\text{/}}} \\{\left( {0.0001\; m^{2} \times 1\mspace{14mu}{{atm}\left( {0.21 - 0.10} \right)}} \right)\;} \\{{{OTR}_{T}\left( {{cc}\text{/}m^{2}\text{-}{day}\text{-}{atm}} \right)} = {908\text{,}000\mspace{14mu}{cc}\text{/}m^{2}\text{-}h\; r\text{-}{atm} \times 24{\mspace{11mu}\;}{hr}\text{/}{day}}} \\{21\text{,}792\text{,}000\mspace{14mu}{cc}\text{/}m^{2}\text{-}{day}\text{-}{atm}}\end{matrix} & (1) \\\begin{matrix}{{Flux}_{02\text{-}{MP}} = {{21\text{,}792\text{,}000\mspace{14mu}{cc}\text{/}m^{2}\text{-}{day}\text{-}{atm} \times 0.0001\mspace{14mu} m^{2}} -}} \\{= {2179\mspace{14mu}{cc}\text{/}{day}\text{-}{atm}}}\end{matrix} & (3)\end{matrix}$

Laser drilling of polyvinyl chloride sheet produces Type Imicroperforations. Therefore, 9, 150-micron perforations, each with aFlux₀₂ of 250 cc/day-atm, are needed to maintain the desired atmospherein the package at 4° C.:2179 cc/day-atm/250 cc/day-atm for each Type I microperforation=9microperforations

Rigid polyvinyl chloride lids were microperforated using a 10-wattlaboratory laser with a scan head (stopped web method). Nine, 150-micronperforations were drilled into each lid. Freshly sliced strawberries(0.454 kg) were placed in containers, microperforated lids were applied,and a band of shrink tape was applied over the flange area to achieve ahermetic seal. Packages were stored at 4° C. for 14 days.

Within 48 hrs, the atmosphere inside the packages had equilibrated tothe desired O₂ and CO₂ levels. The strawberries maintained their colorand turgor, and there was no obvious mold growth on the fruit during a14 day storage period at 4° C.

The present invention has been particularly shown and described withrespect to certain preferred embodiments of features. However, it shouldbe readily apparent to those of ordinary skill in the art that variouschanges and modifications in form and details may be made withoutdeparting from the spirit and scope of the invention as set forth in thefollowing claims. The objects and advantages of the present inventionmay be further realized and attained by means of the instrumentalitiesand combinations particularly pointed out in the appended claims.Accordingly, the drawing and description are to be regarded asillustrative in nature, and not as restrictive.

1. An improved packaging for establishing optimum atmospheric conditionsfor respiring produce, comprising: a non-porous polymeric material; aset of microperforations on said polymeric material, wherein said set ofmicroperforations are drill holes and based on a number and a size ofsaid microperforations, control and maintain said optimum atmosphericconditions within specified O₂ and CO₂ concentrations for said respiringproduce, said optimum atmospheric conditions containing less than about20.9% O₂ and greater than about 0.03% CO₂, wherein said polymericmaterial provides a total O₂ Flux ranging from 150 cc/day-atm to5,000,000 cc/day-atm and wherein each of said microperforations has anaverage diameter between 110 and 400 microns and said set ofmicroperforations are placed in a registered target area on saidpolymeric material, said registered target area being a finite region onsaid polymeric material.
 2. The improved packaging material according toclaim 1, wherein said polymeric material is selected from the groupconsisting of polyethylene, polypropylene, polyester, nylon,polystyrene, styrene butadiene, cellophane, and polyvinyl chloride, inmonolayers, coextrusions, or laminates.
 3. The improved packagingmaterial according to claim 1, wherein said polymeric material isheat-sealable.
 4. The improved packaging material according to claim 1,wherein said polymeric material has a thickness in the range of 0.4 to 8mil.
 5. The improved packaging material according to claim 1, whereinsaid polymeric material provides a total O₂ Flux ranging from 200cc/day-atm to 1,500,000 cc/day-atm.
 6. The improved packaging materialaccording to claim 1, wherein said polymeric material forms a bag. 7.The improved packaging material according to claim 1, wherein saidpolymeric material is a heat sealable film forming a lid.
 8. Theimproved packaging material according to claim 1, wherein said polymericmaterial is formed into a semi-rigid container with a thickness greaterthan 25 mil.
 9. The improved packaging material according to claim 6,wherein said bag is substantially enclosed with a top seal, a bottomseal, and a pair of side seals, and wherein said registered target areais within one-quarter distance from said top seal of said bag.
 10. Theimproved packaging material according to claim 6, wherein said bag issubstantially enclosed with a top seal, a bottom seal, and a pair ofside seals, and wherein said registered target area is within one-thirddistance from said top seal of said bag.
 11. The improved packagingmaterial according to claim 1, wherein said registered target area islocated in an area that prevents occlusion of the microperforations byproduct, labels or other packages.
 12. The improved packaging materialaccording to claim 1, wherein said polymeric material has a CO₂transmission rate that is 2.5 to 5.0 times greater than the O₂transmission rate.
 13. The improved packaging material according toclaim 1, wherein each of said microperforations has an average diameterin the range between 120–160 microns.
 14. The improved packagingmaterial according to claim 1, wherein said polymeric material has a CO₂transmission rate that is 3.4 to 4.0 times greater than the O₂transmission rate.